Ya know before the kids was tried as an adult, they more than likely gave the family the opportunity to plea bargain.
"If he signs a full confession, we'll make sure he gets sentanced as a minor."
If HE feels he was wrongly convicted, he can ask for a retrial. But chances are there was plenty of evidence for him being the killer. Stabbing the brother to death.. That's pretty brutal.
Now, life no parole for non-murder... That's a different thing. But again, it depends on the crime and severity.
"I would rather be exposed to the inconveniences attending too much liberty than to those attending too small a degree of it." Thomas Jefferson